Cut off by northern river (6)
I believe the answer is:
severn
'river' is the definition.
(River Severn)
'cut off by northern' is the wordplay.
'cut off' becomes 'sever' (to sever is to cut).
'by' is a charade indicator (letters next to each other).
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'sever'+'n'='SEVERN'
(Other definitions for severn that I've seen before include "British banker" , "Shrewsbury river" , "River at Worcester" , "Longest UK river" , "England's longest river" .)